Ceramic enamels can be used for a variety of applications, such as decorative coatings for glassware, chinaware, and the like. They also have found use in recent years as colored borders around glass sheets used for automotive windshields, sidelights and backlights. The colored borders enhance appearance and prevent the degradation of underlying adhesives by UV radiation.
In general, these enamel compositions consist mainly of a glass frit, a colorant and an organic vehicle. They are applied to a glass substrate and subsequently fired to burn off the organic vehicle and fuse the frit to the substrate.
Glass sheets for automotive use are generally coated with the ceramic enamel composition and then subjected to a pressure forming process at elevated temperatures. During this treatment the enamel melts and fuses to the glass substrate and the glass is formed into a desired final shape. However, many prior art coatings exhibit a tendency to adhere to the materials covering the forming die, e.g., a fiberglass or metal fiber covered die, because these conventional enamels have a low viscosity after melting and have a tendency to stick to other materials at high temperature. Accordingly, such previous enamels are not suitable for use in glass forming processes in which the heated glass coated with enamel contacts a forming die.
Various proposals have been made in an effort to facilitate the forming of glass sheets with a ceramic enamel coated thereon, and to resist elevated bending or forming temperatures without the enamel adhering to the forming die. For example, U.S. Pat. Nos. 4,596,590 and 4,770,685 propose the addition of a low valent metal oxide powder, e.g., cuprous oxide, to the paint composition to provide a non-stick barrier between the coating and the fiberglass-covered forming die. U.S. Pat. Nos. 4,684,389; 4,857,096; 5,037,783 and EP 490,611 (issued to Boaz), propose adding finely divided zinc metal powder for a similar effect. The use of an iron metal powder is proposed in U.S. Pat. No. 4,983,196 (issued to Stotka).
A purportedly improved anti-stick ceramic enamel composition is proposed by U.S. Pat. Nos. 5,153,150; 5,208,191 and 5,286,270 (issued to Ruderer et al.) wherein a seed powder containing Zn.sub.2 SiO.sub.4 is introduced into an oxide composition with or without the presence of bismuth oxide (Bi.sub.2 O.sub.3).
A further shortcoming of a number of previous ceramic enamel systems is that they employ a lead-containing glass frit. For environmental considerations it is desirable to avoid the use of any lead-containing system.
Also, while several of the above-mentioned enamel systems may perform satisfactorily in conventional glass forming processes, some may not be suitable for use in the newly-developed "deep bend" processes for forming automotive glass. Moreover, the enamel compositions must resist chemical agents that may come into contact with them.
Previously known or proposed enamel compositions suffer from one or more of the deficiencies noted above. In contrast, the present invention provides a ceramic enamel composition that overcomes these shortcomings.